1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to a process cartridge and an image forming apparatus using the photoreceptor, which are applied to copiers, facsimiles, laser printers, direct digital platemakers, etc.
2. Description of the Related Art
At one time, photoreceptors including inorganic materials such as selenium, zinc oxide, and cadmium sulfide were the dominant type of electrophotographic photoreceptors used in electrophotographic image forming apparatuses such as copiers and laser printers. However, at present, almost all electrophotographic photoreceptors are organic photoreceptors, which are more advantageous than the inorganic photoreceptors in terms of load reduction on the global environment, cost reduction, and design freedom. Further, as the importance of production in consideration of global environmental protection increases, the organic photoreceptor is required to change from a disposable item to a durable machine component.
In order to make the organic photoreceptors more durable, various attempts have been made. At present, forming a crosslinked surface layer as disclosed in Japanese published unexamined application No. 2000-66424, and forming a sol-gel cured layer on the surface of a photoreceptor as disclosed in Japanese published unexamined application No. 2000-171990, have generated high expectations. The former has an advantage of scarcely having a crack even when including a charge transport component and increasing yield when produced. In particular, a radical polymerizable acrylic resin can prepare a durable and high-sensitivity photoreceptor. These methods are thought reasonable because they form a crosslinked layer with plural chemical bonds, which is not quickly abraded even when the chemical bonds are partly cut with stress.
The above-mentioned photoreceptors have better durability when including a high-hardness filler. Since coating materials for the surface layer are mostly monomeric components, the coating materials have low viscosity and the high-hardness filler occasionally has a problem with dispersion stability. When the filler does not have good dispersion stability, the photoreceptor has an awkward surface and adversely influences cleanability of a toner. Therefore, the filler needs to increase its dispersion stability in coating materials for the crosslinked surface layer, but this aspect has not been studied enough.
A polymerized (spherical) toner is mostly used as a developing toner for electrophotography because of having advantages of being environmentally friendly when prepared and producing higher quality images.
The polymerized (spherical) toner is a spherical toner without any angulation, and is prepared by chemical preparation methods such as suspension polymerization, emulsion aggregation polymerization, ester elongation polymerization, and solution suspension. The polymerized toners have different shapes according to their preparation methods, and the polymerized toners used in image forming apparatuses have shapes slightly different from a complete sphere. The polymerized toner typically has an average circularity of from 0.95 to 0.99 and shapes factors SF-1 and SF-2 of from 110 to 140. A toner is a complete sphere when having an average circularity of 1.0 and shapes factors SF-1 and SF-2 of 100.
Having a uniform shape, the polymerized toner is likely to hold a uniform charge. Further, the polymerized toner easily includes a wax (5 to 10%). Therefore, the polymerized toner has good developability and transferability, and produces images having good sharpness, image resolution, and tone reproducibility because of scarcely spreading out of an electrostatic latent image. In addition, the polymerized toner has many advantages such as no need for an oil when transferred. However, the polymerized toner has poor cleanability and needs to have more external additives, resulting in occasional killifish-shaped filming on a photoreceptor. Many studies have been undertaken to solve this problem and many suggestions are made in patent documents, etc.
A photoreceptor is typically required to have a low friction coefficient and to keep this even when repeatedly used, such that a polymerized toner has cleanability. For example, Japan Hardcopy Fall Meeting, 24-27, 2001 written by Nobuo Momotake, Akihisa Maruyama, Satoshi Shigesaki, and Hiroe Okuyama, discloses improving cleanability of a polymerized toner by applying a solid lubricant such as zinc stearate onto the surface of a photoreceptor.
When a solid lubricant such as zinc stearate is externally applied to a highly-durable electrophotographic photoreceptor on which the crosslinked radical polymerizable acrylic resin layer is formed, occasionally the surface of the photoreceptor does not accept the solid lubricant. Many photoreceptors have smooth surfaces, and it is thought that this poor acceptability is due to the surface smoothness of a photoreceptor. Japanese published unexamined patent application No. 2007-79244 discloses a method of roughening the surface of a photoreceptor to stably apply a lubricant thereto. Specifically, the surface roughness (Rz-JIS '94) thereof is preferably from 0.4 to 1.0 μm, and a filler is preferably added into a surface layer thereof to maintain a specific surface roughness.
However, the photoreceptors have various shapes of surface roughness even when having the same Rz. For example, even photoreceptors having extremely different distances between concavities and convexities have the same Rz. Even photoreceptors having the same Rz occasionally have rank orders of zinc stearate acceptability. Electrophotographic photoreceptors need special conditions besides Rz to increase acceptability of zinc stearate. The surface roughness of an electrophotographic photoreceptor is an important property, and has been measured by a method specified in JIS B0601 in many cases.
Methods measuring roughness includes a method of measuring arithmetic average roughness, a method of measuring a maximum height (Rmax), a method of measuring ten-point average roughness, etc. However, values measured by these methods run into difficulties when concavities and convexities are considerably distant from each other in a measured range. However, good methods of evaluating roughness have not been available, and parameters showing roughness are being studied as follows.
Japanese published unexamined patent application No. 7-104497 specifies a partition width X based on an average line 2 on a profile curve (1) obtained by measuring a surface profile with a surface roughness measurer to evaluate the surface profile by the number of peaks formed of a pair of peaks (4) and troughs adjacent to each other over the partition width at a unit length (L). An organic photoreceptor is prepared using a substrate prepared under the following conditions.
Partition width X: 20 μm
Unit length L: 1 cm
The number of peaks (4): 100 or less
Japanese published unexamined patent application No. 2002-196645 discloses, for the purpose of producing high-quality images, a cleaning roller applied with a bias voltage separating a charged toner from a photoreceptor located on an upstream side of a cleaning blade, and the photoreceptor has an average ten-point surface roughness Rz of from 0.1 to 2.5 μm.
Japanese published unexamined patent application No. 2006-163302 discloses a method of satisfying the following relations:ΔT>Rz0 nm<ΔT+Rz<5 nmwherein ΔT represents a layer abrasion amount of an electrophotographic photoreceptor per 1 k cycle and Rz represents a surface roughness.
Japanese published unexamined patent application No. 2007-86319 discloses an electrophotographic photoreceptor, the surface of a photosensitive layer of which is roughened. In addition, the roughened surface has a standard deviation of glossiness of 4 or less when measured.
Japanese Patent No. 3040540 discloses a system including a blade, a toner composition, and an unused image forming member including a surface forming a latent image with the toner composition. The surface has a roughness defined by the following formulae:R/ann4>KB(1−σ2)/32 πEt2af andR/ann2<√{square root over ( )}3/8π2·(1+μ2)/μ·KB/Γ·t/af·θwherein R represents an average altitude of convexities on the surface, ann represents half the closest adjacent distance among the convexities, KB represents a volume elastic modulus, σ represents a Poisson ratio of the toner composition, E represents a Young's modulus, t represents an average thickness of flat particles in the toner composition, of represents an average radius of the flat particles, μ represents an average between a toner-blade friction coefficient and a toner-surface friction coefficient, Γ represents a Dupre's work, and θ represents a blade tip angle.
Japanese Patent No. 3938209 discloses a cylindrical electrophotographic photoreceptor including a cylindrical substrate and an organic photosensitive layer formed thereon. In claim 1, the electrophotographic photoreceptor has plural dimple-shaped concavities on its circumferential surface; a ten-point average roughness measured in a circumferential direction of the surface thereof. Rzjis (A) is from 0.3 to 2.5 μm; a ten-point average roughness measured in a bus direction of the surface thereof. Rzjis (B) is from 0.3 to 2.5 μm; an average distance RSm (C) among convexities and concavities, measured in a circumferential direction of the surface thereof, is from 5 to 120 μm; an average distance RSm (D) among convexities and concavities, measured in a bus direction of the surface thereof, is from 5 to 120 μm; a ratio (D/C) of the average distance RSm (D) among convexities and concavities to the average distance RSm (D) among convexities and concavities is from 0.5 to 1.5; and 5 to 50 of the dimple-shaped concavities having a maximum diameter of from 1 to 50 μm and a depth of from 0.1 to 2.5 μm in an area of 10,000 μm2 on its circumferential surface. Further, in claim 2, the Rzjis (A) is limited to 0.4 to 2.0 μm; the Rzjis (B) is limited to 0.4 to 2.0 μm; the RSm (C) is limited to 10 to 100 μm; the RSm (D) is limited to 10 to 100 μm; and the ratio (D/C) is limited to 0.8 to 1.2. In claim 3, the electrophotographic photoreceptor has a maximum height Rp (F) of the circumferential surface not greater than 0.6 μm, and a ratio (E/F) of a maximum depth Rv (E) of the circumferential surface to the maximum height Rp (F) not less than 1.2.
Japanese Patent No. 3938210 discloses an electrophotographic photoreceptor including a substrate and an organic photosensitive layer formed thereon. The electrophotographic photoreceptor has plural dimple-shaped concavities on a surface layer, and 5 to 50 of the dimple-shaped concavities having a maximum diameter of from 1 to 50 μm, and a depth not less than 0.1 μm and a volume not less than 1 μm3 in an area of 100 μm2 on the surface layer. In addition, plural concavities corresponding to the dimple-shaped concavities on the surface layer are formed on an interface between the surface layer and a layer just under the surface layer.
Japanese published unexamined patent application No. 2005-345788 discloses an image forming apparatus including plural image bearers each formed of an electroconductive substrate and a photosensitive layer thereon, forming an electrostatic latent image when irradiated; plural image developers for the plural image bearers, each developing the electrostatic latent image with a developer; and plural cleaners for the plural image bearers, each scraping the surface of the image bearer to remove the developer. At least a pair of the image developers contain developers having the same hue and different brightness. According to the brightness, ten-point average roughness Rz of the image bearer in the initial state is controlled.
Japanese published unexamined patent application No. 2004-258588 discloses an image forming apparatus using an electrophotographic photoreceptor having a ten-point average roughness Rz of from 0.1 to 1.5 μm or a maximum height Rmax not greater than 2.5 μm, a JIS-S hardness of from 70 to 80, and a friction resistance Rf less than 200 gf and greater than 45 gf which is a tension load measured by applying a load of 100 g on a flat belt made of polyurethane having a width of 5 mm, a length of 325 mm, a thickness of 2 mm and a weight of 4.58 g at a contact length of 3 mm and a contact area of 15 mm2 in a circumferential direction.
Japanese published unexamined patent application No. 2004-54001 discloses an image forming method including a process of developing a latent image formed on an electrophotographic photoreceptor with a developer to form a toner image; a first transfer process of transferring the toner image onto an intermediate transferer; a second transfer process of transferring the toner image onto a recording material; and a cleaning process of removing a residual toner on the electrophotographic photoreceptor after transferring the toner image onto the recording material. The electrophotographic photoreceptor has a surface roughness Ra of from 0.02 to 0.1 μm, the intermediate transferer has a surface roughness Rz of from 0.4 to 2.0 μm, and a surface energy reducer is applied to the surface of the electrophotographic photoreceptor.
Japanese published unexamined patent application No. 2003-270840 discloses an organic photoreceptor having a cycle average of surface concavities and convexities ten times or more of a volume-average particle diameter of a toner.
Japanese published unexamined patent application No. 2003-241408 discloses an image forming apparatus including an electrophotographic photoreceptor rotating at a peripheral speed not less than 200 mm/sec and a cleaner. The electrophotographic photoreceptor includes an electroconductive substrate, a photosensitive layer on the substrate, and a surface protection layer on the photosensitive layer. The surface protection layer includes a fluorine-containing particulate resin in an amount of from 35.0 to 45.0% by weight based on total weight of the protection layer, and has a ten-point average roughness of from 0.1 to 5.0 μm, a surface hardness of from 0.1 to 10.0 when measured by TABER abrasion test method, and a surface friction coefficient of from 0.1 to 0.7. The cleaner is an elastic rubber blade having a linear pressure of from 0.294 to 0.441 N/cm against the photoreceptor, a tensile elasticity (Young's modulus) of from 784 to 980 N/cm2, a repulsive elasticity of from 35 to 55%, and includes fluorine-containing particulate resin at the surface. In addition, the image forming apparatus uses a toner having a glass transition temperature (Tg) of from 40 to 55° C.
Japanese published unexamined patent application No. 2003-131537 discloses an image forming method using an image former having the following relationship:d/t×0.01≦Ra0.5wherein d/t represents a flatness of a toner (d is a volume-average particle diameter of the toner and t is a thickness thereof) and Ra (μm) represents an arithmetic average roughness of the image former.
Japanese published unexamined patent applications Nos. 2002-296994 and 2002-258705 disclose an image forming apparatus including an image bearer having convexities and concavities smaller than a volume-average particle diameter of spherized toner on its surface.
Japanese published unexamined patent application No. 2002-82468 discloses an image forming apparatus including an electrophotographic photoreceptor rotating at a peripheral speed not less than 200 mm/sec and a cleaner. The electrophotographic photoreceptor includes an electroconductive substrate, a photosensitive layer on the substrate, and a surface protection layer on the photosensitive layer. The surface protection layer includes a fluorine-containing particulate resin in an amount of from 15.0 to 40.0% by weight based on total weight of the protection layer, and has a ten-point average roughness of from 0.1 to 5.0 μm, a surface hardness of from 0.1 to 20.0 when measured by TABER abrasion test method, and a surface friction coefficient of from 0.001 to 1.2.
Japanese published unexamined patent applications Nos. 2001-265014, 2001-289630, 2002-251029, 2002-296822, 2002-296823, 2002-341572, 2002-296824, 2006-53576, 2006-53577 and 2006-79102 disclose a method of evaluating surface profile using Fourier transform. The Fourier transform can grasp variations frequently appearing in signals as its frequency distribution, but cannot effectively grasp variations not frequently appearing. Further, where the variation is made is not known.
Japanese published unexamined patent application No. 2004-117454 discloses a method of determining a profile curve defined in JIS B0602 from a random point on the surface of a substrate at a length of 100 μm in an axial direction, measuring a position of the profile curve in a longitudinal axial direction at an equally-spaced position on a horizontal direction, determining a dispersion defined in JIS 28101 of the position, and determining a measurement value selected from Ra, Rz and Ry defined in JIS B0601 to evaluate a surface roughness of the substrate.
However, none of the above surface roughness measurement methods can completely evaluate cleanability of image forming apparatuses using a toner having a small particle diameter or a polymerization toner.
Namely, conventional methods using terms such as Ra, Rmax and Rz cannot precisely measure the surface roughness.
Therefore, conventionally, a record chart of surface roughness/profile measurers is saved and the roughness is determined from a cut waveform recorded in the record chart. However, a trend of the record chart needs reading out and skill.
Japanese published unexamined patent application No. 2007-79244 has the following problem. In its Examples, particulate alumina is used. The particulate alumina needs specific film-forming conditions because of having unstable filler dispersibility in coating materials. Particulate polymethylsilsesquioxane also used therein cannot be said to have sufficient acceptability for lubricants because it is thought that a photoreceptor including polymethylsilsesquioxane at its surface has large concavities and convexities and cannot sufficiently bear a solid lubricant.
A coating material for crosslinked surface layer has low viscosity because of mainly including monomeric components. In contrast, since silicon-containing particulate materials such as silica and particulate silicone resins have high dispersion stability in a coating material for crosslinked surface layer, they are more advantageously used among various fillers. However, it was difficult to actually use the silicon-containing particulate materials in the following points. Japanese published unexamined patent application No. 2005-99688 discloses an example using a silicon-containing particulate material in Example 2 mentioned from [0162]. However, even in this Example, the photoreceptor does not have sufficient acceptability for a solid lubricant. It is thought that the photoreceptor has large concavities and convexities on its surface and cannot sufficiently bear a solid lubricant. An additional new technology is needed.
Japanese published unexamined patent application No. 08-248663 discloses dispersing an inorganic particulate material (hydrophobized silica) having an average particle diameter of from 0.05 to 0.5 μm in a photosensitive layer having a surface roughness of from 0.1 to 0.5 μm at a depth of from 0.05 to 15 μm, which is formed on an electroconductive substrate having a surface roughness of from 0.01 to 2 μm.
The dispersed hydrophobized particulate silica increases durability of the photoreceptor and prevents deterioration of image resolution (image distortion) caused by adherence of contaminants such as corona products. The hydrophobized inorganic particulate material repels water (a contact angle thereof is large), but the adherence of corona products cannot be prevented. As disclosed in Japanese published unexamined patent application No. 2004-138643, alumina is used as a filler to avoid image distortion. However, as mentioned above, alumina is difficult to include as is in a crosslinked surface layer.
In an image forming apparatus externally adding a solid lubricant to the surface of a photoreceptor, the lubricant acceptability thereof influences an abrasion speed thereof and cleanability of a toner, impacting printed image quality. At present, technologies sufficiently improving the lubricant acceptability of a photoreceptor having a highly-durable crosslinked surface layer are not yet available.
Japanese published unexamined patent applications Nos. 2004-61359 and 2007-292772 disclose a surface roughness measurement method of sensitively and precisely grasping a local transformation and variation of the surface of an image forming member such as substrates of electrophotographic photoreceptors, in which a profile curve defined in JIS B0601 of the surface of the image forming member is determined, and a multiple image resolution analysis such as wavelet conversions of lines of positional data in a surface roughness direction at equally-spaced positions on the profile curve is performed.
Because of these reasons, a need exists for improvement of acceptability of a highly-durable electrophotographic photoreceptor having a crosslinked surface layer for a lubricant.